In the prior art, different compact and closed press sections are known, which are provided with a combination of press rolls, wherein the press rolls form, as a rule, at least three press nips with one another. Examples of these prior art press sections include applicant's "Sym-Press II" press section as well as the "Sym-Press O" press section (trademarks of Valmet Paper Machinery, Inc.).
In a "Sym-Press II" press section, the first twin-felt nip is formed between a hollow-faced lower press roll and an upper press-section roll. A suction roll forms a second nip with a smooth-faced center roll of the press, and a third press nip is also formed in connection with the center roll.
In a "Sym-Press O" press section, the first twin-felt horizontal nip is formed between a lower hollow-faced roll and an upper press-suction roll, after which the web follows the upper fabric as a vertical run into a second nip. The second nip is formed between a hollow-faced roll separate from the rolls of the first nip and the smooth-faced center roll. A third press nip in connection with said center roll is provided with a felt, is formed by means of a hollow-faced press roll.
In measurements carried out by the applicant, it has been noticed that, e.g. in the "Sym-Press II" press section described above, which is provided with a separate press (fourth press nip), the temperature of the web before the third press nip is about 3.degree.-10.degree. C. higher than before the fourth separate nip. If a steam box is employed (in a manner known in prior art) in connection with the center roll in the "Sym-Press II" press roll, by means of which steam box the temperature level of the smooth faced center roll is raised between the second and third nip, cooling takes place before the fourth nip even to a greater extent that what was stated above. Corresponding cooling of the web also takes place in a "Sym-Press O" press section provided with a separate nip, and in other, corresponding press section concepts.
However, cooling the web before the fourth nip lowers the efficiency of dewatering in the nip. Thus, for the purposes of energy economy, it is well known that dewatering by means of pressing is considerably more advantageous than dewatering by evaporation. Accordingly, all realistic measures should be taken to achieve a maximal dry solids content in the web in the dewatering taking place by pressing in order to achieve an efficient dewatering system.
The reasons for cooling the web in the press nips and during its transfer from the center roll of a press to a separate press include the following. The web is cooled as a result of its movement in air currents. The web is carried into the separate press over a considerably long distance on the top face of a relatively cool press felt. Before the fourth nip, the web reaches direct contact with the colder smooth faced upper roll of the press on a certain advance covering sector.
With respect to the prior art, reference is made to U.S. Pat. No. 3,655,507, as well as to U.S. Pat. No. 3,560,333. From these publications, it is known in prior art to heat the press felt and the web before the web enters into the dewatering press nip. The heating arrangements known from said publications are, however, not satisfactorily suitable for use when attempts are made to raise the temperature level of the web to be pressed efficiently after the compact press roll combination before the web is passed into the separate nip.
Even though, in the following, the invention will be described with reference to the applicant's "Sym-Press II+separate nip" press section, it should be emphasized that the present invention is also suitable for use in other, corresponding press sections, such as the applicant's above-mentioned "Sym-Press O" press section and corresponding press constructions of other manufacturers of paper machines.